1. Field of the invention
The present invention relates, in general, to refrigerating circuit devices which control the driving frequency of a variable capacity compressor in response to an air conditioning load in a room to be conditioned. In particular, the invention relates to a refrigerating circuit device which carries out a defrost operation in which the driving frequency of the compressor decreases to remove the frost on the internal heat-exchanger in a cooling mode.
2. Description of the prior art
In air conditioners, frost may adhere and build up on an evaporator (internaI heat-exchanger) because of decreace of the refrigerant evaporating temperature, which occurs because of a shortage of refrigerant, or a clogged air filter during cooling operation.
To prevent the formation of frost on the evaporator, in an air conditioner which uses a variable capacity compressor, the driving frequency of the compressor forcibly reduces when the temperature of the evaporator decreases below a prescribed value. One example of this operation is shown in FIGS. 1 and 2. When the temperature Tc of an evaporator detected by a suitable sensor decreases below a prescribed value T1 (from the A area into the B area shown in FIG. 1), a timer begins its counting operation. The driving frequency f of the compressor reduces periodically by a prescribed value every time the timer achieves a prescribed time value t1, as shown in FIG. 2. When temperature Tc increases above the prescribed value T1 (from the B area into the C area), the driving frequency f at which the temperature Tc exceeds the prescribed value T1 is maintained in the C area. After that, when temperature Tc exceeds a second prescribed value T2 (from the C area into the A area), the maintaining of the driving frequency f is cancelled, and a normal operation in which the driving frequency of the compressor varies in response to the air conditioning load in the room is carried out in the A area. The timer counting operation also is cancelled, as shown in FIG. 2. At this time, if the temperature Tc enters from the A area into the B area. and returns from the B area to the A area within the prescribed time tl, the operation of reducing the driving frequency f of the compressor is not carried out.
In the above-described prior art operation, frost builds up on an evaporator in a small air conditioning load during cooling, in spite of the decreasing of the driving frequency described above. This is because the temperature Tc is relatively low, e.g., 6.degree. C., in a small air conditioning load. Under such condition, the driving frequency f approachs zero Hz, and the compressor stops. When the temperature Tc gradually increases and returns to the A area, the compressor re-starts and the normal operation is again carried out. However, if the second prescribed value T2, which is a boundary value between the C area and the A area, is set to a lower value in the above-described operation, the frost still remains on the evaporator when the temperature Tc exceeds the second prescribed value T2 (A area). If the prescribed value T2 is set to a higher value to avoid such a disadvantage, the temperature Tc cannot exceed the second prescribed value T2 (A area) in a normal air conditioning load or in a large air conditioning load.